1. Technical Field
The present invention relates to an elongating apparatus for a glass base material such as an optical fiber preform that includes a top chamber made of an extendable multilayer cylinder.
2. Related Art
An optical fiber preform (glass rod) serving as the material for optical fiber is manufactured by being heated and elongated from a large glass base material for use as an optical fiber. As an example of a glass base material elongating apparatus, Japanese Patent Application Publication No. H11-060259 discloses an elongating apparatus that has an extendable top chamber. According to the method described in Japanese Patent Application Publication No. H11-060259, the overall height of the elongating apparatus is kept low by using the extendable top chamber, and therefore a miniaturized elongating apparatus can be used for a large base material.
Furthermore, Japanese Patent Application Publication No. 2011-116592 discloses an elongating apparatus with a top chamber made of glass. According to Japanese Patent Application Publication No. 2011-116592, the heat accumulated in the tapered portion at the top of the glass base material can be restricted by using the glass top chamber, thereby improving the outer diameter control during the latter half of the elongation. Japanese Patent Application Publication No. 2011-116592 describes using a single glass tube as the top chamber, and Japanese Patent Application Publication No. H11-060259 describes using a multilayer cylinder that can extend as the top chamber.
In order to decrease curvature in a glass rod obtained through elongation, Japanese Patent Application Publication No. H09-030827 and Japanese Patent Application Publication No. 2005-104763 describe methods for holding the glass base material at a constant XY position during elongation.
In an elongating apparatus with an extendable top chamber, it would be possible to use a splicing fitting to fix the top lid and the flange provided on the top of the extendable top chamber. However, since there is little distance between the heating furnace and the top lid and flange during the latter portion of the elongation, the top lid becomes warped due to the high temperature and the flange can break or cause friction, resulting in damage.